dt-bindings: serial: stm32: update rts-gpios and cts-gpios

Update rts-gpios and cts-gpios:
- remove max-items as already defined in serial.yaml
- add a note describing rts-gpios and cts-gpios usage with stm32

Document the use of cts-gpios and rts-gpios for flow control in STM32 UART
controller. These properties can be used instead of 'uart-has-rtscts' or
'st,hw-flow-ctrl' (deprecated) for making use of any gpio pins for flow
control instead of dedicated pins.
It should be noted that both cts-gpios/rts-gpios and 'uart-has-rtscts' or
'st,hw-flow-ctrl' (deprecated) properties cannot co-exist in a design.
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Link: https://lore.kernel.org/r/20210106162203.28854-6-erwan.leray@foss.st.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/st,stm32-uart.yaml

@@ -50,11 +50,14 @@ properties:
     minItems: 1
     maxItems: 2
 
-  cts-gpios:
-    maxItems: 1
-
-  rts-gpios:
-    maxItems: 1
+# cts-gpios and rts-gpios properties can be used instead of 'uart-has-rtscts'
+# or 'st,hw-flow-ctrl' (deprecated) for making use of any gpio pins for flow
+# control instead of dedicated pins.
+#
+# It should be noted that both cts-gpios/rts-gpios and 'uart-has-rtscts' or
+# 'st,hw-flow-ctrl' (deprecated) properties cannot co-exist in a design.
+  cts-gpios: true
+  rts-gpios: true
 
   wakeup-source: true